Paint formulations for producing weathering-resistant and chemical-resistant, high-flexibility coatings on plastics are of great interest owing to the increasing use of plastics for high-value applications, for example in vehicle construction. A central problem there, however, is the provision of polyols which give the cured coating maximum flexibility in conjunction with high hardness, excellent low-temperature flexibility and very good resistance properties, such as resistance to weathering, to tar and to solvents and other chemicals.
Paints based on polyester polyols as described for example in EP-A 166 962, EP-A 318 800, EP-A 571 829 or EP-A 650 992 are distinguished in relation to polyacrylate polyols in particular by very high flexibility, particularly at low temperatures down to about −45° C., by high hiding power in conjunction with effective levelling, and by high gloss. Problems, however, are their solvent resistance and chemical resistance, which are no more than moderate, and a deficiency in their surface hardness. Polyacrylate polyols, on the other hand, do not attain flexibility properties comparable with polyester polyols.
It was therefore an object of the present invention to provide new polyol components which combine the known advantages of the polyester polyols with those of the polyacrylate polyols and lead to coating systems which at one and the same time meet the following requirements:                1. Yellowing-free film surfaces on exposure to heat (thermal yellowing): the paint film must withstand exposure to heat (120° C.) over a period of at least 20 h without substantial yellowing (increase in ΔE<3).        2. Low-temperature elasticity: even at −20° C. the film must not tear when the plastic is deformed.        3. Resistance to solvents and other chemicals and to tar spots: the paint film must withstand exposure to solvents and other chemicals and to a special test solution containing tar and must do so without damage.        4. Effective gloss retention and high scratch resistance: the paint film on the plastics parts ought to have maximum durability and, under mechanical exposure and effects of weathering, should not become scratched more rapidly and/or lose gloss more rapidly than the paint film on the metal bodywork (DIN 67 530, ISO 2813).        5. No later embrittlement on weathering: even after prolonged weathering the film must not show any loss in low-temperature elasticity.        6. Low drying temperature in combination with extremely short drying times, for reasons of energy saving and to allow even heat-sensitive plastics parts to be painted. Additionally, drying temperatures <80° C. allow the systems to be used in automotive refinish paints, so that they are suitable not only for original (OEM) finishing.        7. Paint film hardness: in combination with a low-temperature elasticity of −40° C. the paint film ought to attain a hardness of >80 s (König pendulum damping). At a low-temperature elasticity of −20° C. the paint film hardness ought to reach a value of at least 125 s.        
Furthermore, the polyol components ought to have a very low viscosity (<3000 mPas/23° C.) in combination with a very high solids (>70% by weight), in order to allow them to be used in low-solvent coating systems.